Photoelectron multipliers which utilize the photoelectric effect and photodiodes which utilize pn junctions have hitherto been used as photosensors. Such photosensors are employed in a broad range of applications that include not only light sensing, but also, as solid-state imaging devices, image sensors for civilian use and on-board applications in vehicles.
Lately, given current trends toward smaller sizes and lighter weights, and also the importance placed on low cost and decorative design, organic photosensors that use organic materials in the photoelectric conversion layers are being developed and have been attracting attention.
Organic photosensors are constructed of, for example, a photoelectric conversion layer, charge (hole, electron) collecting layers, electrodes (anode, cathode) and an optical filter.
Of these, the photoelectric conversion layer and the charge-collecting layers are generally formed by a vacuum deposition process. However, vacuum deposition has drawbacks in terms of, for example, its complexity as a mass production process, the high cost of the equipment, and the efficiency of material utilization.
In light of these drawbacks, water-dispersible polymeric organic conductive materials such as PEDOT/PSS are sometimes used as coating-type materials for forming hole-collecting layers. However, because these are aqueous dispersions, the complete removal of moisture and the control of moisture reabsorption are difficult, which tends to accelerate device deterioration.
Moreover, there remain a variety of challenges facing the use of aqueous dispersions of PEDOT/PSS in mass production. Namely, because such dispersions are prone to solids agglomeration, defects readily arise in applied films made thereof and the coating equipment has a tendency to clog or corrode, in addition to which the applied films leave something to be desired in terms of heat resistance.
Also, in photosensors, an external electrical field is generally applied on account of the need to maximize the light-receiving properties (see Patent Document 1).
Although applying an external electrical field is likely to improve the light-receiving sensitivity and the response speed, the injection of holes and electrons from the electrodes arises, resulting in an increase in the dark current.
Because this increase in dark current is a factor that lowers the detection sensitivity, in order to create devices having a high light-receiving sensitivity, suppression of the dark current while continuing to exhibit a high photoelectric conversion efficiency is important.